The use of polyolefins, such as polyethylene, has become quite prevalent. A particularly useful application of polyolefins has been for a class of photographic supports. The photographic support generally includes a paper base material having a polyolefin coating. The polyolefin coating provides a very smooth surface when thin layers of a silver halide emulsion, are desired to be coated on the photographic paper.
The polyolefin coating can include an optical brightener and typically, a fluorescent whitening agent to make the white areas of the support even brighter. The optical brightener compensates for the yellow cast produced by the absorption of short-wavelength light such as violet to blue by the polyolefin. Typical optical brighteners fluoresce upon irradiation with ultraviolet light emitting visible light, typically bluish in hue, replacing the light that would have been lost and thereby enhancing the brightness of the support. Optical brighteners for use in such fields as photographic printing materials must absorb ultraviolet light, especially in the region from 280 to 405 nanometers (nm), and re-emit such light so as to enhance the brightness of the print. Desirably, the optical brightener has stability to temperatures as high as 310.degree. C. to 330.degree. C., so that it may be incorporated into the polyolefin during an extrusion process or film forming process and in extruding the polyolefin onto the paper based material.
It is desirable that the optical brightener be non-migrating so that it remains in the polyolefin coating and does not exude as a surface film on the polyolefin. Such exudation not only gives rise to a non-uniform brightness of the reflection surface, but also readily transfers to any other surface coming in contact with it. For example, a brightener transferred to the back side of the adjacent support layer when wound in roll can adversely affect subsequent coating and finishing operations of the photograph. This can result in the final product having a poor quality and performance.
It is recognized in the art that the addition of optical brighteners to polyolefins has been less than satisfactory. For example, U.S. Pat. No. 3,449,257 describes 2,5-bis(benzoxazolyl)thiophenes, such as Uvitex OB.RTM. available from Ciba Specialty Chemicals, as non-migrating brighteners for hydrophobic polymers, but were found to undergo bleeding and yellowing from a polyolefin film in U.S. Pat. No. 5,173,397
In an effort to improve the compatibility between hydrophobic or non-polar polyolefins and optical brighteners, U.S. Pat. Nos. 4,794,071 and 4,859,539 issued to Tomko, et al. on Dec. 27, 1988, and Aug. 22, 1989, respectively, disclose a mixture of optical brighteners that when incorporated into a pigmented polyolefin layer of a photographic support minimizes exudation of the brightener at the polyolefin surface. The mixture of optical brighteners include certain fluorescent bis (benzoxazolyl)stilbenes. U.S. Pat. No. 4,794,071 teaches that single component optical brighteners exhibit severe exudation when compared to the optical brightener mixture described in U.S. Pat. No. 4,794,071.
Accordingly, there is a need for the incorporation of an optical brightener into a non-polar polyolefin that will exhibit the good compatibility with the polyolefin and not exude as a surface film on the polyolefin.
Additionally, there is a need for a method of incorporating optical brighteners into hydrophobic or non-polar polyolefins which will enhance their brightening efficiency.